Continuing advances in disk drive storage capacities and speed have been greatly aided by concomitant improvements in the optical read/write head positioning systems they employ. Generally, a light source, holograms, lens and mirrors make up such optical positioning systems and provide the information needed by the drive's electronics to exactly position the head.
One such optical system is found in the a:drive, a disk information storage drive made by O. R. Technology Inc. of Campbell, Calif. A hologram will be found at the heart of the a:drive's optical positioning system. The hologram looks like a tiny piece of square shaped glass which contains a two dimensional image, usually developed by a computer program. The program and the hologram pattern accounts for light source efficiency, the optical characteristics of the various components involved in positioning the read/write head used in the drive system, the thermal dynamic character of those components and the drive's geometric characteristics.
A laser diode provides a source of coherent light that is used to illuminate the hologram. As light passes through the hologram apertures, it forms a pattern of six spots on whatever diskette is then resident in the drive. Three of the spots are used to position the head for 120 MB ultra high density (UHD) media and three are used for positioning the head with respect to 720 KB or 1.44 MB floppy diskettes. Each set of three spots actually contains two "striped" spots and a single spot that doesn't have any stripes. The so-called striped spots are themselves actually made up of several individual pin points of light which give the spot an overall appearance of being striped.
As light leaves the hologram, a lens focuses the spots and then a mirror splits the light in two directions; up and onto the surface of a UHD disk or down onto the surface of a encoder for use with respect to a high density disk. On the UHD media, the several stripes of light are projected over an equal number of the disk's data tracks and provide tracking information which is used to accurately position the read/write heads.
The non-striped spot, known as a modulation or "M" spot, is theoretically a steady, unvarying spot of incident light that measures the modulation and reflectivity of the UHD media surface as the intensity of the light returned from the modulation spot. That information is gathered by an appropriate detector and used by the system to compensate the striped spot readings for incident stray energy impinging on them. Unfortunately, stray or side lobe energy resulting from generation of the striped spot closest to the modulation spot was falling thereon causing the modulation spot to take on the ripple intensity characteristics of the nearby striped spot, thereby distorting the information the modulation spot conveys back to the servo positioning system. This prevents proper spacing of the spots, causing them to be kept further apart than is optimum for the servo system being used. Thus, the design choices in such prior art optical positioning systems was to move the spots closer together in spite of the undesirable stray energy that would impact the modulation spot and accept the resulting servo positioning errors or move the spots further apart to minimize or negate the stray energy problem and accept reduced servo tracking performance due to servo tracking errors.
Much of the spurious light noise that occurs is the result of edge diffraction caused by the sharp edges of the hologram's apertures. This is a known problem and efforts have been made in the past to compensate for such effects by rounding, tapering, serrating or smoothing a hologram's aperture edges. Unfortunately, such compensation efforts did not prove useful in helping solve the problems of spot adjacent noise described above.
On the other hand, holograms have been used in the past to correct for other unwanted effects in optical systems. For example, an optical head with two holographic elements adapted to provide primary dispersion compensation and secondary collimating or objective element compensation is described in U.S. Pat. No. 4,776,652 granted Oct. 11, 1988 to C. Ih. While not concerned with compensation for aperture edge induced diffraction errors, this patent does show the use of holograms to correct optical system errors. U.S. Pat. No. 5,121,371 issued on Jun. 9, 1992 and U.S. Pat. No. 5,563,868 which issued on Oct. 8, 1996, both show an optical servo system for a magnetic disk wherein light is passed through slits in a holographic optical element (HOE). No mention is made of aperture edge diffraction problems or compensation therefor although use of a HOE as part of a temperature compensation scheme is described.
Also, U.S. Pat. No. 5,315,417 issued on May 24, 1994 to G. Moss et al teaches the use of a hologram element as a spatial filter to block unwanted wavelengths. In a similar vein, U.S. Pat. No. 5,530,565 which issued on June 25, 1996 to H. Owen describes the use of a HOE as part of an optical bandpass filter. However, none of these prior art patents taught any manner of compensating for HOE aperture edge diffraction created noise.